Activity coefficients in electrolyte solutions and molten salts

Different definitions of activity coefficients in aqueous electrolyte solutions are compared with each other. It is most convenient to define activity coefficients within the molality mode related to the “ideal dilute standard”, which is indeed the best choice for dilute solutions. In the range of very high salt concentration (> 20 mol kg⁻¹), definitions, which use the mole fraction, are more suitable or even necessary. We describe four definitions, which are suitable for electrolyte solutions in a single solvent. Furthermore, the consequences of the use of a mole fraction in the definitions of activity coefficients and excess functions are brought into focus.